Cardiac pacemakers are well known in the art. Such devices apply electrical pulses to one or more chambers of the heart. The energies of such applied electrical pulses are selected to be above the pacing energy stimulation threshold of the respective heart chamber to cause the heart muscle of that chamber to depolarize or contract. Depolarization of the heart muscle of the respective chamber in turn causes the respective chamber to contract. In this manner, the required pumping action of the heart is sustained.
If an applied pulse has an energy below the pacing energy stimulation threshold of the respective chamber, the pacing pulse will be ineffective in causing the heart muscle of the respective chamber to depolarize or contract. As a result, there will be failure in sustaining the pumping action of the heart. It is therefore necessary to utilize applied pacing pulse energies which are assured of being above the pacing energy stimulation threshold.
However, it is also desirable to employ pacing energies which are not exorbitantly above the stimulation threshold. The reason for this is that cardiac pacemakers are usually implanted beneath the skin of a patient and hence are battery powered. Using energies that are too much above the stimulation threshold would result in early depletion of the battery and hence premature cardiac pacemaker replacement.
It is therefore desirable to ascertain the pacing energy stimulation threshold of a heart chamber to be paced. A pacing energy may then be selected which is above the threshold to assure reliable pacing but no so high as to unduly deplete the battery.
As is well known in the art, the stimulation threshold of a heart chamber can, for various reasons, change over time. Hence it is further desirable to have the cardiac pacemaker periodically and automatically determine the pacing energy threshold. In this way, the variations or changes in stimulation threshold can be accommodated to both assure reliable pacing and extended battery life.
When a pacing pulse is effective in causing depolarization or contraction of the heart muscle, it is referred to as "capture" of the heart. Conversely, when a pacing pulse is ineffective in causing depolarization or contraction of the heart muscle, it is referred to as "lack of capture" of the heart.
An electrogram (EGM), as is also well known in the art, is the electrical activity of a heart muscle. The electrical manifestation of lack of capture in a heart muscle is typically a negative deflection in the electrogram baseline. This is referred to as polarization (POL). The electrical manifestation of capture in a heart muscle is typically an exaggerated biphasic deflection in the EGM. This is generally referred to as the evoked response plus polarization (ER+POL).
When a cardiac pacemaker performs a pacing energy stimulation threshold search or test, it is essential that there not be lack of capture because the patient still requires cardiac rhythm management. Generally, these searches are performed by applying a succession of test pacing pulses at a basic rate. The energy of each successive pacing pulse is reduced by a known amount and capture is verified following each pulse. If a test pulse fails to capture, a backup or safety pulse is applied to sustain heart activity. The energy of the test pulse to last capture is then used as a basis for determining the energy threshold. In these methods, capture may be verified by detecting T-waves, mechanical heart contraction, changes in cardiac blood volume impedance, or another signature of a contracting chamber.
Other methods are also known for providing automatic pacing energy stimulation threshold determinations which provide successive pairs of pacing pulses. Each pair of pulses includes a primary pulse and a secondary pulse. The secondary pulse is used to determine an estimate of the polarization and to provide safety pacing. In these techniques, the pulses in the delivered pair have the same amplitude and pulse width so as to provide the same pacing energy. The pulses of each pair are timed such that, if one pulse captures, the other pulse will provide a measure of polarization. The polarization waveform is subtracted from the evoked response plus polarization waveform to determine if capture occurred. Unfortunately, if the capture threshold is below the energy of the two identical pulses of a pulse pair, no provision is made for providing pacing until the next pulse pair. In essence, the primary and secondary pulses are unsuccessful in capturing the heart chamber and as a result, the heart drops a beat. In determining the stimulation threshold of the ventricle, this occurrence is unacceptable.